jQuery Grid Plugin - jqGrid

hi to all ! First of all thanks for this great plugin!,

i´m using jqgrid 3.8.1 with form edit and i have 3 columns ( between others)  in grid : "amount", "cost" and "total", i want that when i add a new row column "total" set automatically when i left the field Total empty in the form, as the sum of "amount" and "cost".

So total = amount + cost.

Is this possible?,

Thanks in advance,

best regards,

                             nicolás.